In the geological environment, the primary industrial source of chromium is the mineral chromite, which can be represented by the ideal formula FeO.Cr2O3. In practice, FeO can be partially substituted by other elements such as MgO, CaO, MnO and Cr2O3 by Fe2O3 and Al2O3. These substitutions are at the origin of different types of chromites distinguished, among other things, by their chrome to iron ratios. In the geological environment, the chrome to iron ratios of chromites vary from 1.3 to 4.0 in many stratiform or podiform deposits. Chromites possessing chrome to Iron ratios higher than 3, are rare in nature.
Chromites are employed in the production of ferrochromium, a master alloy in the stainless steel industry. The primary process for the production of ferrochromium from chromites is described by the general reaction: metal oxide+reductant+energy(ferro)metal+reductant oxide. The production of ferrochromium is an energy-intensive process and is generally conducted in an electrical furnace. Ferrochromiums can be divided in three classes based on their carbon content: high carbon ferrochromium containing between 4 to 10% carbon; medium carbon ferrochromium containing between 0.5 to 4% carbon; low carbon ferrochromium containing less than 0.5% carbon. The chrome to iron ratio of the chromite ore used as a feed to the furnace, controls the chromium content of the ferrochromium. The value of the ferrochromium is mainly based on its chromium and carbon contents. The highest prices are obtained for ferrochromium showing high concentration in chromium and low carbon content. Similarly, the chromites economic values are set by their chrome to iron ratios: a chromite with a Cr/Fe ratio of 1.5 being worth less than a chromite with a Cr/Fe ratio of 4. The economic value of these chromium-enriched chromites is increased in their use as enriched product directly and as feed for ferrochromium production.
Hence, there is a need for a method for increasing the chrome to iron ratio of a chromite ore. Methods for achieving this goal have been described.
European Patent No. 0 096 241, by Robinson and Crosby, describes the chlorination of chromites mixed with coke by Cl2 at a temperature ranging between 1000° and 1100° C. The chromites are completely transformed into chlorides and volatilized. The iron chlorides and chromium chlorides are separated according to their respective boiling points. This specific process leads to the formation of pure CrCl3.
South African Patent No. 96/4584 by Lalancette, Bergeron, Bossé, Clerk teaches the chlorination of chromites by Cl2 in the presence of air, no reductant being used. The process is described by two reactions.2FeO.Cr2O3+3Cl2=2FeCl3(g)+2Cr2O3+O2  1.2FeCl3+3/2O2=Fe2O3+3Cl2  2.The combination of these two reactions results in:4FeO.Cr2O3+4Cl2+O2=4Cr2O3+2Fe2O3+4Cl2  3.According to this process, the iron is selectively chlorinated and transformed in gaseous FeCl3. While FeCl3 is still in the reaction vessel, this product is rapidly transformed in Fe2O3 via reaction No. 2. This result in the production of a chromite showing an increase in its chrome to iron ratio with a simultaneous formation and precipitation of Fe2O3 as hematite in the chlorination reactor. After the chlorination step, the reactor is drained and the hematite is dissolved in concentrated HCl leaving a residue of enrich chromite.
U.K. Patent No. 1,567,841 by Sowden and Rigg teaches the chlorination of Cr2O3.xH2O by CCl4 below 600° C. The resulting product is CrCl3. The reaction at the base of this process is:2Cr2O3.5/2H2O(amorphous)+11/2CCl4=4CrCl3+11/2CO2(g)+10 HCl(g).Following the chlorination reaction CrCl3 is dissolved in diluted HCl.
Thermodynamic and kinetic studies of the chlorination of chromites and associated oxides such as FeO, Fe2O3, and Cr2O3 have also been published by Martirosyan (1978 a, b; Arm. Khim. Zh. 31, pp. 93-99; 100-106) as quoted by Kanari, Gaballah, and Allain (1999, Metallurgical and Materials Transactions B, 30B, pp. 577-587) for instance. These studies were centered on thermodynamic and kinetic considerations and do not teach how to apply these principles to a workable and optimized method. They do not teach the use of a catalyst to increase the efficiency of the reactions.
Chlorination as a general metallurgical approach has also been described. Johnstone, Weingartner and Winsche (1942, J. Am. Chem. Soc., 64, pp. 241-244) observed the formation of a eutectic point when studying the binary system ferric chloride(FeCl3)-sodium chloride. Cook, and Dunn (1961, J. Phys. Chem., 65, pp. 1505-1511) refined the phase diagram and presented evidence for the formation of NaCl.FeCl3. Bezukladnikov, Tarat and Baibakov (1974, Zr. Prikl. Khim. 47, pp. 1722-1725); and Zhao, Tian and Duan (1990, Metallurgical Transactions B, 21B, 131-133) studied the solubility of chlorine in different molten salts. These authors concluded that the presence of FeCl2 in molten salts increases by two orders of magnitude the speed of the chlorination reactions. They attributed this increase to the catalyst role played by FeCl2 according to the reaction: FeCl2(melt)+0.5Cl2(gas)=FeCl3(melt). The actual partial pressure of chlorine at the reaction site decreases rapidly causing decomposition of FeCl3 with the liberation of chlorine at the reaction sites. FeCl2 reacted with external chlorine thus regenerating FeCl3. This system increases chlorine diffusion and acts as a transport procedure for chlorine at the reaction sites and accelerates the chlorination process.
It is apparent from the foregoing that known methods for chlorinating chromites result either in the production of CrCl3 because of the temperatures used (i.e. 1000° C.) or in the formation of secondary hematite (Fe2O3) that has to be leached by concentrated. HCl in order to produce chromites showing high chrome to iron ratios. Furthermore, thermodynamic and kinetic studies on chlorination of iron have not incorporated the effect of the catalyst role played by FeCl2, FeCl3 in the presence of molten salts and they do not integrate the required systems for the set up of a commercial process such as those taking account environmental requirements. Furthermore, these studies do not teach how to avoid potential problems related to the consumption of chlorine by others oxidic constituents occurring in the natural spinels structure of chromites and in other silicated phases associated with the ore.
Investigations on the chemical compositions of chromites from the Menarik Complex, Bay James, Québec, Canada, have showed that the oxidic components of the chromite ores are highly variable. Table 1 shows chemical analysis performed by an electron micro-probe of chromite grains extracted from Cr-3 chromite showing of the Menarik Complex. These results indicate important variations in the major oxides phases on a grain-to-grain basis. The average chemical composition of the Cr-3 mineralized zone is reported in the Table 2 with the heading Starting ore.
TABLE 1Chemical analysis of chromite grains by electronmicro-probe, Menarik Cr-3 chromiteSampleMgOAl2O3SiO2TiO2V2O3Cr2O3MnOFeOCr/FeNo.%%%%%%%%N/ACr3-267.7417.230.001.280.5845.591.2326.361.52Cr3-273.9817.590.000.500.3541.811.8233.951.08Cr3-372.9316.450.000.000.3644.871.1934.191.16Cr3-352.5817.410.791.020.9540.811.9732.141.12Cr3-291.8315.670.000.000.8742.771.7236.011.05Cr3-281.753.100.980.000.0044.721.2748.170.82Cr3-442.735.920.820.720.7942.100.9346.000.81Cr3-431.485.230.490.001.2541.062.6847.810.76N/A: not applicable.
There thus remains a need to develop an effective method for the selective extraction of iron from heterogeneous natural chromites in such a way that other oxides such as CaO, MgO, MnO, SiO2, TiO2, Cr2O3 are left substantially unaffected by the method and through means that are secure for the environment. There also remains a need for a method able to extract the iron without the need to dissolve the hematite coatings on chromites with concentrated HCl, a complicated and expensive procedure. There also remains a need for a method including a catalyst component to accelerate the chlorination process and efficient environmental and recycling systems.
It is an object of the present invention to provide an improved method of increasing the chromium/iron ratio of chromites.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art.